This invention relates to electric power transmission and distribution equipment, and more particularly to devices for joining sections of certain types of electric power transmission cables.
Electric power is often transmitted at voltages exceeding 50 kV in order to reduce power losses caused by the resistance of the conductors. Traditionally, such high-voltage conductors have been suspended high above the ground from towers or other suitable supports in order to isolate them from the ground and from other objects where a high difference of potential would exist between the conductors and the objects. In such applications, the conductors are electrically insulated from the supports by suitable insulator apparatus, and from everything else by the air present in the region around the conductor. As is well known, an electric field surrounds the conductor. Because air has a relatively low dielectric strength, the conductors must be separated from other objects by a relatively large distance to prevent the electric field gradient in the region between the conductor and the object from exceeding dielectric strength of the insulating air.
The above-ground transmission of electric power via suspended conductors may be inappropriate for certain applications. In some cases, the requirement that the conductors be spaced far from other objects is inconsistent with existing or planned land use patterns. In other cases, aesthetic considerations preclude the use of the large towers or other supports required. One possible solution to this problem would be to locate air-insulated conductors underground in suitable vaults, but the need to maintain adequate physical separation between each conductor and other conductors and surrounding objects would require huge vaults and renders this solution economically infeasible.
For these and other reasons, systems have been designed to permit electricity to be transmitted at high voltages through suitable cables having configurations which do not require large physical spacing between the conductor and other objects. In one such cable configuration, a center conductor is surrounded by a layer of an appropriate solid dielectric material, such as polyethylene. The solid dielectric layer is, in turn, surrounded by a conductive shield. The center conductor, the solid dielectric, and the conductive shield are concentrically disposed. The center conductor has a substantially circular cross section. In order to avoid skin effect, the center conductor may comprise several groups of smaller conductor strands. Such groups are arranged as sectors of the circular center conductor cross section. The conductive shield may be formed as a tubular layer of partially conductive material having one or more drain conductors running along the outside surface of the layer. A corrugated metal tube or other suitable armor may be provided around the conductive shield to provide physical protection against damage to the cable.
The solid dielectric layer is formed from a suitable material having a high dielectric strength to minimize the distance required between the center conductor and the shield for a given operating voltage. This reduces the amount of material required to construct the dielectric layer and all other layers disposed radially outward from the dielectric layer. Accordingly, the weight, cost, and overall diameter of the cable is minimized.
The electrical stress in the region surrounding the center conductor of such cables is high. Special precautions are necessary to avoid electrical breakdown at any place where either the center conductor or the shield conductor are terminated or deformed, because any variation in the geometry of the conductors will cause the stress distribution in the region to change. If the mechanical configuration of the conductors is such that the electrical stress is concentrated, the stress may exceed that which the dielectric medium between the conductors can withstand. In particular, when a section of a cable is joined to another section, the center and shield conductors are necessarily physically discontinuous. Although it is theoretically possible to attach the respective conductors of the two cable sections to produce a configuration within the joint region which is mechanically and electrically identical to that of the cable, it is nearly impossible to achieve this in practice.
Accordingly, when sections of solid-dielectric cable are joined, the joint is typically constructed in a structure designed to reduce the electrical stress in the region of the joint so that the mechanical elements required to create the joint do not produce excessive electrical stress concentrations. Such joints are typically immersed in a suitable container of insulating fluid (e.g. oil), having a high dielectric strength in order to reduce the separation required to avoid breakdown. In addition, conductor arrangements are chosen carefully to avoid sharp edges and other configurations which produce large concentrations of electrical stress and thereby promote breakdown.
It is highly desirable to minimize the number of joints between sections of underground cables. Electrical losses may occur at the joints. In addition, a significant amount of skilled labor is required to build and install the joints, and the material cost of each joint is relatively high. Although cable manufacturers attempt to produce sections of cable which are as long as possible, production processes and other constraints limit the length of a section of cable which can practically and economically be manufactured. In addition, cables used in power transmission applications are relatively heavy, and this weight along with the need to transport the cable from the manufacturer's plant to the place of installation, impose a further limit on the maximum length of a cable section.
A significant problem in the design and construction of underground electric transmission facilities is to join sections of underground cable in a way that provides excellent electrical conductivity and high mechanical strength. Excellent electrical conductivity is important because resistance in the joint causes a portion of the electrical energy flowing through the joint to be converted to waste heat. High mechanical strength is importance because the cable sections may be subject to substantial amounts of mechanical stress. In particular, the cable sections are subject to expansion and contraction as the temperature of the cable and surrounding environment varies. Variations in the cable temperature may occur in part as a result of changes in the amount of current being carried through the cable. These mechanical loads may be sufficient to separate the cables at the joint or otherwise disrupt the joint unless suitable provisions are made to constrain them. In particular, when both cable sections contract, very large tensile loads may be placed on the joint.
Existing joint structures have a variety of disadvantages in underground solid-dielectric cable applications. Several existing techniques are known for joining the center conductors of the two cable sections, including crimping and welding. A problem with both of these techniques is that they produce waste products such as conductive particles and contaminants. As previously mentioned, the joint between cable sections is typically formed within a suitable enclosure which contains an appropriate dielectric fluid such as an insulating oil or a gas such as sulfur hexafluoride (SF6). Any conductive particles or contaminants which may remain after the joint has been constructed may be attracted to regions of high electrical stress and the adjacent conductors.
When such a particle comes into contact with a conductor, it forms a sharp protrusion into the fluid. Such sharp protrusions cause concentrations of electrical stress which may exceed the dielectric strength of the fluid. In addition, such concentrations tend to attract other particles, resulting in progressively longer, breakdown-promoting conductive chains. Also, the byproducts of the welding and crimping processes tend to contaminate the environment in which the process is performed. As a result, these processes are not suitable for constructing a joint in high cleanliness environments, such as "clean rooms."
Because the welding or crimping process must be performed as one of the first steps in joint construction, it is difficult or impossible to use certain types of prefabricated joint components in welded or crimped joints. Stress control cones, corona shields, and certain support insulators are preferably constructed as monolithic structures without seams or other structural discontinuities. These components must surround the center conductor, and therefore, if they are monolithically constructed, they must be installed before the welding or crimping step is performed. However, once the components have been installed on the cable, they physically interfere with access required to perform the actual welding or crimping. These components also interfere with installation of the tape layers normally required in welded or crimped joints. Thus, many preferred joint components cannot be used with conventional welded or crimped joints.
Another problem with welded and crimped joints is that the connection cannot be conveniently disconnected as required for maintenance or reconstruction. An additional problem with welded joints is that although welding the conductors provides a good electrical and mechanical connection, it requires precise longitudinal and axial alignment of the conductors, and this alignment is difficult to achieve in field installations.